This invention relates to a reactive ion etching (RIE) process for controllably etching semiconductor materials including silicon bulk and silicide and polycide films to obtain vertical wall profiles. More particularly, the invention is a RIE process characterized by good profile control, high etch selectivity to insulator masks and little toxicity and corrosiveness stemming from use of a novel combination of etchant gases.
RIE is a well-known dry etching technique which finds extensive utility in the fabrication of semiconductor integrated circuits. A major application of RIE is to form deep trenches in a silicon substrate to surround an active or passive semiconductor device and electrically isolate the device. U.S. Pat. No. 4,104,086 issued to Bondur et al and assigned to the present assignee describes the details of the trench isolation process. The trenches are etched after masking portions of the substrate surface with a material (e.g., silicon dioxide or silicon nitride or a combination thereof) which has a lower etch rate than that of silicon. The trenches are then filled with a suitable material such as oxide, polyimide or polysilicon. Another major application of RIE is to define extremely narrow conductor lines (e.g., a gate for field effect transistor or interconnect structure for connecting devices on a substrate) from a deposited film of a refractory metal silicide, polycide (silicide/polysilicon sandwich), etc. In both these applications, it is imperative that the etched surfaces have substantially vertical profile and the (submicron) width of the trench/line be under tight control, lest severe constraints are imposed on the design and fabrication of viable and operable integrated circuits.
A variety of etchant gases and their mixtures have been used in the prior art for RIE of semiconductors. U.S. Pat. No. 4,214,946 to Forget et al and assigned to the present assignee discloses use of a gas mixture consisting of 10 parts of SF.sub.6 and Cl.sub.2 and the remainder an inert gas (He) for RIE of silicon or polysilicon. While this reference achieves an etch rate ratio (ERR) of Si:SiO2 exceeding 40:1, it is basically directed to etching shallow (about 0.5 .mu.m depth) trenches. Moreover, this process has the fundamental disadvantages of high toxicity and corrossive power which necessitate highly expensive and complicated safety measures during etching. Another disadvantage is that additional process steps are required for making an etch mask of SiO.sub.2 because photoresist etch masks cannot withstand these potent gases.
U.S. Pat. No. 4,264,409 to Forget et al and assigned to the present assignee, discloses RIE of silicon or polysilicon using a gas mixture of SiF.sub.4, Cl.sub.2 and an inert gas (Ar or He). This process suffers from the same drawbacks as the afore mentioned U.S. Pat. No. 4,214,946.
U.S. Pat. No. 4,530,736 issued to Mutter and assigned to the present assignee discloses RIE of Si using 95-99% of CCl.sub.3 F or CCl.sub.2 F.sub.2 in combination with 1-5% of oxygen using a photoresist mask. This patent teaches that oxygen is added to the other gas to increase the etch rate of the photoresist and oxygen does not have significant effect on the etch rate of silicon.
U.S. Pat. No. 4,475,982 issued to Lai et al and assigned to the present assignee discloses RIE of deep trenches in silicon in an atmosphere of CCl.sub.2 F.sub.2 and Ar to etch the lightly doped layers of silicon and CCl.sub.2 F.sub.2 and oxygen to etch the heavily doped layers. The switch over to the second atmosphere is made to avoid lateral etching or "blooming" in the heavily doped layers of the silicon substrate.
U.S. Pat. No. 3,880,684 issued to Abe et al describes a semiconductor prepared by continuously etching at least two types of silicon compound layers, such as, SiO.sub.2, Si.sub.3 N.sub.4 or polysilicon which are formed on a silicon substrate. A freon gas plasma is used for etching so that the two types of silicon compound layers are continuously etched in a sloped form with undercutting, as occurs in conventional wet chemical etching.
The article entitled "Dry Process Technology (Reactive Ion Etching)" by J. A. Bondur, J. Vac. Sci. Tech., Vol 13, No. 5, pp. 1023-1029, Sept./Oct. 1976 discloses a variety of gases (CF.sub.4, CCl.sub.2 F.sub.2 --Freon 12, etc.) and gas combinations (CF.sub.4 +O2, CF.sub.4 +O2+He, etc.) for RIE of Si, SiO.sub.2, Si.sub.3 N.sub.4, etc. against a photoresist mask. Bondur concludes that even if the resist mask is square to the wafer surface, it does not insure that a vertical-walled image will result in the etched material since the plasma reacts with the photoresist.
U.S. Pat. No. 4,447,290 issued to Matthews teaches adding Freon 12 to an etchant gas mixture of SF.sub.6 and oxygen during plasma etching of polysilicon on a Si.sub.3 N.sub.4 layer for facilitating detection of the end point of polysilicon etching.
U.S. Pat. No. 4,465,553 issued to Hijikata et al discloses patterning by etching silicon or a compound thereof using a gaseous mixture of SF.sub.6 and C.sub.2 ClF.sub.5.
U.S. Pat. No. 4,473,435 issued to Zafiropoulo et al discloses plasma etching of a polysilicon film to expose a dielectric underlayer on a silicon substrate by using a gas mixture of SF.sub.6 (or CF.sub.4 or NF.sub.3) and Freon (C.sub.2 ClF.sub.5).
U.S. Pat. No. 4,473,436 issued to Beinvogl discloses RIE of a polycide layer using a preferred gas mixture of SF.sub.6 and Cl.sub.2. The patentee states that "the invention can also be practiced by using gas mixtures which contain fluorohydrocarbons that are substituted with chlorine atoms" (i.e. fluoro-chlorocarbons) such as CClF.sub.3, CCl.sub.2 F.sub.2 and mixtures thereof. Further, a carrier gas, preferably an inert gas, such as helium, "can also be utilized in the practice of the invention".
U.S. Pat. No. 4,330,384 issued to Okudaira et al discloses plasma etching of silicon with a gas mixture containing SF.sub.6 and at least one of O.sub.2, NH.sub.3, N.sub.2, CF.sub.4 and CH.sub.4.
U.S. Pat. No. 4,455,193 issued to Jeuch et al discloses simultaneously etching photoresist and oxide layers using a mixture of CHF.sub.3 (or CF.sub.4) and O.sub.2. Other disclosed etchant gases are SF.sub.6 and a mixture of CF.sub.4 and oxygen.
U.S. Pat. No. 4,380,489 issued to Benivogl discloses plasma etching of polysilicon using SF.sub.6 and He reactive gas mixture.
U.S. Pat. No. 4,374,698 issued to Sanders et al discloses plasma etching a Si.sub.3 N.sub.4 or SiO.sub.2 layer using a gas mixture which contains a fluoride compound (CF.sub.4 or CHF.sub.3) and a compound which contains a halogen other than a fluoride (such as CF.sub.2 Cl.sub.2).
U.S. Pat. No. 4,589,952 issued to Behringer and assigned to the present assignee discloses RIE of deep trenches in silicon against a triple photoresist-Si.sub.3 N.sub.4 -photoresist mask using CF.sub.4 etchant ambient containing a low fluorine concentration to obtain a substantially vertical trench wall profile.
Despite the plethora of prior art on RIE etchant gas systems, there exists a strong need for an etchant gas chemistry which is capable of consistently and reliably providing deep and narrow silicon trenches with controlled wall profiles. A similar need exists with respect to patterning fine and ultrafine conductor lines with vertical surface profiles. These needs are particularly acute due to the insatiable demands imposed by the requirements of increased device density on a chip. Coupled to the chip density demands is the safety-at-workplace and environmental considerations making it imperative to avoid altogether any toxic or corrosive gas system. The prior art gas systems which utilize Cl.sub.2 as a component of the etchant gas fails to meet these requirements due to its inherent corrosive and toxic nature. The gas systems which do not use Cl.sub.2 tend to be incapable of providing consistently and reliably controlled wall profiles. Unacceptable trench contour aberrations such as trench sidewall bowing are characteristic of these systems.
Over and beyond the needs stipulated above, there exists a need to controllably alter the shape or pitch of isolation trenches to suit the particular trench-filling material and facilitate reliable trench isolation. For example, for filling trenches with an organic material such as polyimide, it is desirable to etch trenches having a negative pitch (i.e., the bottom of the trench is wider than the top) since, then, the organic will flow down along the trench surfaces and achieve capillary action to fill the trench without forming voids in the organic fill. While filling the trench with polysilicon, for example, a trench profile with a positive pitch (narrower at the bottom than at the top) is desired to prevent formation of large voids in the polysilicon-fill. The prior art failed to address these demands.
Accordingly, it is an object of the invention to provide a process for etching a semiconductor material to obtain controllable wall profiles thereof.
It is another object of the invention to provide an RIE gas system which has a high etch selectivity of a semiconductor against an insulator mask.
It is a specific object of the invention to provide a RIE process for etching trenches in a semiconductor material by manipulating the composition of the component gases in the etchant gas system and/or the RIE process parameters.